The problem with that is the "factory teams" enter only to win for PR purposes so they spend an absolute mint. And because of that spend, they either quit as soon as they win ("Mission Accomplished") or after a few years of not-winning they then pull the plug (Honda, Toyota, BMW).
This is what killed FIA Group C Sports Cars - they tweaked the formula to favor the factories, which made it difficult for the privateers to compete. So once the factory teams left, there were no cars and the series folded.
Strong privateer teams like Williams and McLaren were the real foundation of F1 for the 1980s and 1990s along with "factory privateer" teams like Ferrari and Lotus (I'd consider the current McLaren to be a "factory privateer"). They're successful and large enough to survive the manufacturers coming to play so when those manufacturers leave, there is still a solid core of teams left to keep it going.